[RFI] Quiet Shack PC Systems?

[Ian White, G3SEK]

Deni wrote:
>I'm always intrigued by the photos of other peoples shacks with the PC and
>monitor mounted right in among the radio gear.
>
>I have a conventional tower and CRT monitor which creates nasty QRM in my HF
>receivers, I also have a laptop with TFT screen which is not too bad but
>it's switched mode PSU/Charger is chronic anywhere near my radio.
>
>I'm interested to know how others solve the PC / Radio interface  and
>maintain low interference levels.

There's been plenty of good advice already, and the main question to ask 
is *why* your receiver is so sensitive to something like a small 
switchmode PSU held near to it?

What happens if you disconnect the antenna? If the problem goes away, it 
means that you're picking up the noise on your feedline in common mode, 
and then it is coming back down into the RX in differential mode. 
Alternatively, very similar effects may be taking place in your ground 
system.

You probably have both a mains ground connection and some kind of "RF" 
ground connection. In that case all ground currents will divide between 
those two paths, so you will have transmitted RF flowing through the 
mains ground, and also computer noise currents flowing along your RF 
ground.

The solutions are to clean up your antenna feedlines as others have 
advised, and be as disciplined as Tom is about the grounding.

At the very least, separate your mains feeds into the "radio side" and 
the "computer side". Then feed each side separately through its own 
*three-wire* mains filter that includes a ground choke. This strategy 
will keep RF out of the mains, and it also breaks up any local ground 
loops. A prime suspect for a ground loop is the triangle between the 
radio, the PC, and the RS232 interface cable between the PC and the 
radio.

The good news is that the same package of precautions will reduce noise 
pickup, reduce risks of RF feedback, reduce risks of causing RFI, and at 
least help reduce the risks of damage by lightning-induced mains 
surges... because all of those problems actually have very similar 
origins, in uncontrolled ground currents and/or common-mode currents on 
the feedline.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek